1. Field of the Invention
The present invention relates to a dual light source optical recording and playback system which comprises a first light source, which is a semiconductor laser for irradiating tracks of a recording medium with a recording spot light to produce pits as information units, and a second light source for irradiating the tracks of the recording medium with a reading spot light.
2. Brief Description of the Related Art
A dual light source optical recording and playback system has an advantage of approximately doubled recording speed over a single light source system which twice scans a recording medium. The effective recording speed is about doubled because of the so-called verifying function. To verify a recording with a dual light source system, a pit recorded on the track of a recording medium with a recording spot light from a semiconductor laser as a first light source can be played back by a playback spot light from a second light source to thereby confirm that the pit has been properly recorded, thus eliminating the necessity for twice scanning the track with spot light as required in a single light source system.
Two formats are known for recording information on a card-type recording medium which is used in association with a dual light source optical recording and playback system, one being illustrated in FIG. 6 and the other being illustrated in FIG. 7.
Referring first to FIG. 6, a first format using gaps between sectors is explained below.
As illustrated in FIG. 6 (1), information is recorded in four sectors A to D on one track of the recording medium. The sectors A to D, respectively, consist of sector information regions al to dl and data regions a2 to d2. As shown in FIG. 6 (2), which is an expanded view of the area encircled by the broken line in FIG. 6 (1), sector A includes recorded pits Pl (black dots), a gap G between sector A and sector B, and unrecorded pits P2 (white dots). The change in the interval between pits within the respective sectors A to D represents data lengths consisting of bits which are the constituent unit of digital data, "1" or "0".
When new pits P2 are to be added in sector B on the track of such a recording medium, the end of sector A is first detected and, with the interposition of gap G, the sector information region b1 and data region b2 are then recorded.
In continuous playback of sectors A to D, the playback signal is interrupted at ga G between any two of the respective sectors A to D. Therefore, the timing (synchronization) of playback signal reading is disturbed. To solve this problem, the playback system is arranged so that synchronization is sought upon detection of the first playback signal for each of sectors A to D.
To increase the amount of information that can be recorded in a medium and avoid the disturbance of timing at the junction between sectors, there has been proposed a format without any gap G between sectors (hereinafter referred to as "continuous write format"), as illustrated in FIG. 7.
As shown in FIG. 7 (1) and (2), this continuous write format has no gap between any two of the respective sectors A to D but, instead, pits P1, ..., P2..., etc., are continually recorded. In this arrangement, it is no longer necessary to seek timing at the point of transition between any two of the respective sectors A to D and the timing, once established within any given track, is retained as it is, thus permitting playback of information in all the sectors.
However, in order to record, for example, pits P2 in sector B after sector A in which pits Pl have already been recorded, the writing of pits P2 in sector B must be performed in good timing with the recording of pits Pl in sector A so that the necessary synchronization may be obtained in playback. For this purpose, it is necessary to provide a pit corresponding to a mark or pattern indicative of the end of each sector of A to D, viz. a "sector end pattern" and, for additional writing, to record the first pit P2 (referred to as pit bPl) for sector B in the position timed with the final pit Pl (referred to as pit aPl) in the sector end pattern for sector A.
If the above dual light source optical recording and playback system is so designed, for assuring an increased effective recording speed, that the recording spot light from a semiconductor laser precedes the playback spot light from second light source on the track with respect to the direction of travel of the recording medium, then it is impossible to verify that the recording spot light is located at that position of pit bP2 because the sector end pattern has not been thoroughly played back with the playback spot light yet, even if the recording spot light arrives at the position of the first pit bP2 in sector B which is to be thereby recorded.
For the above reason, in the case of the continuous write format, it is advantageous to arrange the system so that the playback spot light precedes the recording spot light on the track with respect to the direction of travel of the recording medium. Such an arrangement, however, cancels out the advantage of the dual light source system in regard to improving the effective recording speed.
Furthermore, if the system is so arranged that, with the recording spot light preceding the playback spot light for taking advantage of the dual light source system, the axis of playback spot light is located closer to the axis of recording spot light so that when the recording spot light arrives at the position of pit bP2 in sector B, the pit aPl in sector A will be played back with the playback spot light to complete the detection of the sector end pattern. The light output from the first light source, which is a semiconductor laser having large output power, tends to interfere with the playback spot light and, hence, prevent accurate playback. This means that the distance between the two spot lights must be accurately controlled. However, it is impossible to design a recording and playback system that will assure the constant distance unaffected by changes in ambient temperature, aging and other internal and external factors.